Page 238 - Wastewater Solids Incineration Systems

200 Wastewater Solids Incineration Systems

4.0 INSTRUMENT AND CONTROL SYSTEMS
BACKGROUND AND TERMINOLOGY

4.1 Telemetry
The definition of telemetry is “to measure from a distance.” A physical property,
state, status, or quantity is measured, converted to an electric signal, and transferred
to another (remote) location. The transmitting means can be any medium capable of
serving as a connecting link: copper wire, glass fiber, electromagnetic waves in any
spectrum or routing, either directly to the receiver, or perhaps bounced through
space by satellite.

4.2 Data Acquisition Systems
Data acquisition (DAQ) is the process of collecting data from digital and analog mea-
surement sources through automation. Depending on industry focus, DAQ defini-
tions vary. A pharmaceutical researcher may need high-speed, high-accuracy temper-
ature and pressure measurements to analyze a new experimental process. A trucking
company may want to observe locations of its vehicle fleet. A dairy may want to
query production capabilities, a product being processed, current inventory of prod-
ucts, and status of shipping of those products. The wastewater utility business is a
blend of these needs and typically considers DAQs fundamental components of a
SCADA system. The typical arrangement provides data gathered from remote loca-
tions via a combination of links of the type mentioned previously.

4.3 Direct Digital Control and Distributed Control System

The common definitions of direct digital control (DDC) and DCS take their birth in
the generation of the “mainframe” computer, and are defined based on that classic
perspective.

4.3.1 Direct Digital Control
In DDC, all process control calculations and functions were managed or calculated in a
mainframe, or central computer, which communicated to field terminals or units to
directly control them. Communications were through proprietary communication hard-
ware and protocols, transmitted through direct cabling links. Depending on the genera-
tion of hardware, dedicated wire pairs sent signals directly. Later, wire pairs were still
used, but modules shared (multiplexed) several signals on one wire pair. The modules
were known as multiplexers (MUXs) and demultiplexers (deMUXs). Communications